The gly we use on the farm gets very hot in the shed each summer, and below zero during the winter. I’ve not noticed any deterioration year to year.
Below is some very good info from the University of Montana:
The main concern when storing herbicides in cold conditions is that if the liquid expands upon freezing, the container holding the pesticide may crack or rupture. The simplest solution to this problem is to avoid excess pesticides that require winter storage. If over-winter storage is necessary, the following information, adapted from the 2005 North Dakota Weed Control Guide, gives the minimum storage temperature to avoid risk of reduced herbicide activity.
No storage temperature restriction: Metolachlor products, EPTC, Surpass, Achieve, Maverick. Most dry formulated herbicides in DF or WDG formulations.
Do not store below 40 degrees Fahrenheit:
Assert, Avenge, Campaign, Command, Discover, Extreme, LI-700, Nortron, Prowl, Pursuit Plus, Sonalan, trifluralin.
Do not store below 32 degrees Fahrenheit
Agri-Dex, Basagran, Far-Go EC, Flexstar, Goal, paraquat, Grazone P D, Hyvar, Kerb, Liberty/ATZ, Lorox DF, Poast, Pramitol, Progress, Puma, Pursuit, Quest, Raptor, Redeem, Reflex, Reglone, Stinger, Thistrol, Ultra Blazer, Velpar.
Do not store below 20 degrees F Fahrenheit
Fusilade DX, Plateau, Ro-Neet, Weedar 64.
Do not store below 16 degrees Fahrenheit:
Camix and Lumax.
Do not store below 10 degrees Fahrenheit:
Amitrole T, Arsenal, Curtail/M, Crossbow, Dakota, Fusion, glyphosate, Rodeo, Roundup.
Do not store below 3 degrees Fahrenheit:
Atrazine 4L, Low Vol ester, Bronate Advanced, bromoxynil, bromoxynil Atrazine, Shotgun.
When the winter is over, herbicides should always be checked before they are used. Liquid products that have become separated, crystallized or coagulated should be placed in a warm area (about 70 degrees F) for several days, during which the containers should be inverted or shaken periodically. Usually, the warm temperatures and agitation are enough to redissolve the crystals into the solvent. If the solution does not redissolve, it probably should not be used.